Method for detecting and/or preventing grind burn

ABSTRACT

The present invention provides a method of detecting and preventing grind burn from developing on a gear. The method includes performing acoustic emission testing while the gear is being ground during a grinding operation. The grinding wheel is evaluated during an eddy current test to detect material buildup on the grinding wheel which could cause grind burn. In addition, the method includes collecting swarf from the gear during the grinding operation and inspecting the swarf for an indication of grind burn.

BACKGROUND

The present invention relates to a grinding operation, and inparticular, to a method of detecting and/or preventing grind burn on aworkpiece being ground.

The design and manufacture of bearings, gears, shafts and many othersurface hardened components in modern automotive and aerospaceindustries pose significant challenges. These components require specialattention in choosing the correct parameters for heat treatment as wellas for subsequent machining processes. The latter, if carried outinaccurately, may reduce the surface hardness and diminish thecompressive surface stresses after surface hardening. Accurate andcontinuous control of machining processes such as grinding is essentialin today's production of these components.

Grinding is a machining process used in the manufacture of high accuracycomponents to achieve the required tolerance. Compared with othermachining processes, grinding requires a very large energy input perunit volume of material removed. The majority of this energy isconverted to heat, which is concentrated in the surface layers of thematerial, within the grinding zone. As such, a sharp increase in thelocalized temperature within the surface can occur. Gears and othercomponents that are hardened and subsequently ground can be subjected tosurface tempering of these localized areas known as “grind burns.” Theseverity of the damage, i.e., grind burn, will depend on the temperaturethe workpiece surface attained when ground. In a gear, for example, agrind burn can lower the surface hardness, lower the contact fatiguelife of the gear, and cause microcracks in a burnt tooth, whichnegatively affects the fatigue life of the gear.

There are several factors that contribute to the generation of grindburns. Such factors can include 1) a high stock removal rate duringgrinding; 2) unexpected increase in stock removal from a tooth surfacedue to nonuniform heat treat distortion; 3) high grinding wheelhardness; 4) imbalance of grinding wheel; 5) infrequent dressing of thegrinding wheel; and 6) insufficient coolant for removing generated heat.In a conventional process control method, grind burns are detected afterthe grinding operation. There are two primary conventional methods forinspecting a gear, for example, for grind burns: 1) a destructive methodbased on microhardness reading of the surface below the burnt area; and2) a non-destructive method such as nital etching. The destructivemethod for inspecting gears requires the gear to be destroyed andtherefore renders it unusable. This method is clearly disadvantageousbecause not all gears can be tested, and the gears which are not testedmay suffer damage that is not detectable.

On the other hand, nital etching is currently considered the industrystandard for inspecting gears for grind burns. Nital etching comprisesthe following steps: 1) cleaning the gear and then dipping the gear innitric acid with 3%-5% alcohol or water; 2) rinsing the gear with water;3) dipping the gear in alcohol; 4) bleaching the gear with hydrochloricacid in 4%-6% alcohol or water; 5) rinsing the gear again with water; 6)neutralizing the gear with an alkali solution (minimum pH of 10); 7)rinsing the gear a third time with water; 8) dipping the gear inalcohol; and 9) applying an oil with rust preventative to the gear.After the etching procedure, the gear is visually inspected for evidenceof grind burns under a light source of 200 footcandles (ftc) minimum. Agear that has a grind burn can have a dark gray, blue, or blackappearance, whereas a gear that is free of grind burns can have a lightgray or light brown appearance. A limited amount of grind burn on a geartooth may be acceptable, but only if the tooth is part of anon-fracture-critical gear or if the grind burn does not extend into acritical area of the tooth.

There are several disadvantages to nital etching. First, nital etchingcan reduce the size of the gear. For example, approximately 0.003 mm ofmaterial can be removed from the gear each time the etching process isperformed. Any portion of the gear that requires a tight tolerance whichshould not be exposed to nital etching must be masked to avoid stockremoval (which requires an additional step in the nital etching processdescribed above). A second disadvantage with nital etching is theresulting appearance of the gear. There may be areas of discoloration onthe gear as a result of nital etching. Processes for removing thediscoloration may cause stock removal or surface texture changes.Another disadvantage with nital etching is corrosion of the gear. Whileit is possible to add corrosion protection to the gear, this requires anadditional step to the above-described nital etching process. A fourthdisadvantage is hydrogen embrittlement when atomic hydrogen enters thehardened steel or other alloys. Hydrogen embrittlement may cause a lossin ductility, load-carrying ability, and/or cracking. Catastrophicbrittle failures are also possible. Other disadvantages with nitaletching include environmental considerations, safety concerns, increasedcosts, and lead time. Also, the quality of the inspection of a gear orpart after nital etching depends on the visual capability, skill, andawareness of the inspector performing the inspection.

In addition, not all manufactured parts are required to be inspected forgrind burns using the nital etching process. According to industrystandard ANSI/AGMA 2007-C00, which specifies standard procedures andrequirements for the detection and classification of localizedoverheating on ground surfaces by chemical etch methods, there is no“specific acceptance or rejection criteria” contained therein forinspecting ground parts. In some instances therefore only a certainpercentage or quantity of parts made are inspected. As such, apercentage of parts being made are never tested for grind burns.

Other non-destructive methods for detecting grind burns include MagneticBarkhausen Noise (MBN) and X-ray diffraction. MBN measures residualstress in the gear, but this method has difficulty identifying “goodquality” gears from “poor quality” gears. On the other hand, the X-raydiffraction method is expensive and time-consuming. Another detectionmethod is to shot peen the surface of the gear. If the surface is soft,the method detects this softness due to the texture of the gear. Thetest is subjective, however, and relies on visual inspection foridentifying grind burns.

What is needed therefore is an improved method of detecting andpreventing grind burns on a ground workpiece which overcomes thedisadvantages of the prior art and which can be implemented for testingall ground components being made.

SUMMARY OF THE INVENTION

The present invention provides a method for detecting and/or preventinggrind burns on a ground workpiece such as a gear. In an exemplaryembodiment, the method determines whether a grinding wheel is properlydressed before a grinding operation. The method includes placing a probein contact with the grinding wheel and measuring with the probe aninduced signal in the grinding wheel. The measured induced signal iscompared to a threshold, and if the measured induced signal is greaterthan the threshold, it is inferred that the grinding wheel needs to bedressed or replaced. Moreover, if the measured induced signal exceedsthe threshold, a presence of material buildup is detected on thegrinding wheel. As the probe is placed in contact with the grindingwheel, the probe induces an electric field in the grinding wheel. As theelectric field is induced, the probe detects the induced signal in thegrinding wheel.

In a different embodiment, a method is provided for detecting grind burnduring a grinding operation. The method includes removing material froman object during the grinding operation. The material that is removedfrom the object is collected and inspected for an indication of grindburn. Filter paper, for example, can be positioned substantially beneaththe object for collecting the material, or alternatively, a magnet orother similar device can collect the material. As material is collectedon the filter paper, a correlation can be made between the location ofthe material collected on the filter paper to the location on the objectfrom which the material is removed. The collected material can beinspected by an instrument at at least 173× magnification or greater.The instrument can be a camera, microscope, or other similar device.Indications of grind burn can include discoloration or a change inthickness of the collected material. If there is discoloration or achange in thickness of the collected material, grind burn may bedetected on the object.

In another embodiment, a method is provided for detecting and/orpreventing grind burn on a gear. The method includes grinding the gearwith a grinding wheel during a grinding operation. During the grindingoperation, an acoustic emission signal produced by the grinding ismeasured with a sensor. An electric field is induced in the grindingwheel and unwanted conditions that cause grind burn are detected. In oneform of the method, a probe can be placed in contact with the grindingwheel thereby inducing a signal therein. The probe can measure theinduced signal and compare the induced signal to a threshold. If themeasured induced signal exceeds the threshold, it can be concluded thatthe grinding wheel has material buildup from the gear and the grindingwheel needs to be dressed or replaced.

In another form of the method, the acoustic emission signal is comparedto a threshold, and if the measured signal exceeds the threshold, adetermination is made that too much material is removed from the gear.Moreover, grind burn can be detected if the measured acoustic emissionsignal is greater than the threshold. In addition, if the measuredacoustic emission signal is greater than the threshold, a preventativemeasure can be implemented by suspending the grinding operation.

In a different form of the method, swarf which is removed from the gearduring the grinding operation is collected. The swarf can be collectedby filter paper, for example, which is positioned substantially belowthe gear. The collected swarf can be inspected for an indicia of grindburn such as discoloration or a change in thickness of the collectedswarf. If discoloration or a change in thickness is inspected, grindburn is detected on the gear.

In an alternative embodiment, a large-scale production method isprovided for making gears. The method includes establishing tolerancesfor the amount of material removed from stock to form the gears. Agrinding wheel is selected that maximizes cutting efficiency andrequires infrequent dressing. Also, the number of passes the grindingwheel will make for removing material from the stock is determined. Themethod also includes establishing a threshold amount of coolant flow tobe dispensed to the grinding wheel and stock during the grindingoperation. The stock is ground by the grinding wheel during the grindingoperation and gears are made from the stock. A condition favorable forgenerating grind burn on the gears is determined before, during, andafter the grinding operation. If such a condition is determined, one ormore parameters of the grinding operation is adjusted to eliminate thecondition and the steps of grinding the stock with the grinding wheel,making gears, and determining a condition favorable for generating grindburn are repeated.

An advantage of the inventive method is that grind burns can be detectedand prevented before, during, and after the grinding operation. Duringthe grinding operation, for example, the acoustic emission generated ismeasured and compared to a threshold. If the measured acoustic emissionis greater than the threshold, it is inferred that grind burn is likelybeing generated and the grinding operation can be suspended for furtherevaluation. Likewise, after the grinding operation, swarf collectedduring the grinding operation is inspected for indicia of grind burn.Therefore, even if the acoustic emission generated during the grindingoperation does not detect grind burn, analyzing the collected swarfafterwards may suggest otherwise. As such, the inventive method includessafety nets for detecting grind burns.

The inventive method also includes a process for monitoring thecondition of the grinding wheel. As will be described below, studiesshow that almost 15% of the damage suffered by a gear during a grindingoperation is due to an improperly dressed grinding wheel. Thus, thepresent invention provides a method that detects material buildup on thegrinding wheel that, if not properly removed before a subsequentgrinding operation, can lead to grind burn on a gear or other objectbeing ground.

Another advantage of the inventive method is that every manufacturedgear is tested for grind burn. More importantly, the grinding of eachgear tooth is monitored for grind burn during the grinding operation.This allows immediate detection of grind burn and does not rely solelyon visual inspection. Moreover, the inventive method can be implementedusing tooling and resources available in the same manufacturing facilityin which a gear or other ground workpiece is made. This can provide costsavings and test results much sooner than conventional detectionprocesses which in some instances require finished parts to be shippedto an off-site test facility for testing.

BRIEF DESCRIPTION OF THE DRAWINGS

The above-mentioned aspects of the present invention and the manner ofobtaining them will become more apparent and the invention itself willbe better understood by reference to the following description of theembodiments of the invention, taken in conjunction with the accompanyingdrawings, wherein:

FIG. 1 is a flowchart of an embodiment of a process control system formanufacturing a gear;

FIG. 2 is a flowchart of an embodiment for detecting and/or preventinggrind burn;

FIG. 2A is a flowchart of an embodiment for acoustic emission detection;

FIG. 2B is a flowchart of an embodiment for eddy current testing of agrinding wheel;

FIG. 2C is a flowchart of an embodiment of a swarf analysis process

FIG. 3 is a perspective view of an embodiment in which a gear isundergoing an acoustic emission testing procedure;

FIG. 4 is a schematic view of the acoustic emission testing setup ofFIG. 3;

FIG. 5 is a top view of an embodiment of a grinding wheel undergoingeddy current testing;

FIG. 6 is a schematic view of the eddy current testing setup of FIG. 5;

FIG. 7 is a perspective view of an embodiment of filter paper;

FIG. 8 is a perspective view of filter paper positioned on a gear holderfor collecting swarf during a grinding operation; and

FIG. 9 is a perspective view of swarf collected during a grindingoperation being inspected by a camera.

Corresponding reference numerals are used to indicate correspondingparts throughout the several views.

DETAILED DESCRIPTION

The embodiments of the present invention described below are notintended to be exhaustive or to limit the invention to the precise formsdisclosed in the following detailed description. Rather, the embodimentsare chosen and described so that others skilled in the art mayappreciate and understand the principles and practices of the presentinvention.

The present invention relates to a process control method of detectingand/or preventing grinding burns on a ground workpiece during and aftera grinding operation. One of the purposes behind process control methodsis to control the output of a specific process. In the case ofmanufacturing gears, for example, the process control method or systemis designed to establish parameters for each step in the manufacturingprocess to ensure the manufactured gears are made substantially thesame.

With reference to FIG. 1, an exemplary embodiment of a process controlmethod is provided. While other process control methods may includeadditional or fewer steps for manufacturing a gear, the embodiment ofFIG. 1 provides a method 100 of eight parameters or conditions which canaffect the quality of a ground workpiece. In block 102, the method 100seeks to minimize off center conditions or runout. The pitch diameter ofa gear, for example, can be 3 inches from tooth-to-tooth. However, therecan also be localized areas where the diameter is slightly smaller orlarger (e.g., 2.995″ or 3.005″). In these circumstances, the gear hasrunout of 0.005″ from the centerline of the gear. If the diameterexceeds a runout threshold, there can be unbalanced stock removal duringa grinding operation. To reduce or minimize runout, machines, tools, andgages used for machining the workpiece must meet certain specifications.In addition, the quality of the tooling used for holding the workpiece,the setup, and repair of the machines and tooling are routinely checkedfor runout problems.

In block 104, the process control method further includes ensuring andmaintaining the quality of the workpiece material. The quality andpreparation of the workpiece material can influence the results ofdimensional changes in the workpiece after carburizing and hardeningprocesses, and most importantly, the susceptibility of the workpiece tosurface tempering during gear grinding, for example. Quality refers tothe workpiece, and in particular, to its chemistry, grain structure,microstructure, reduction ratio, and the like. In a process in which agear is manufactured, the quality can refer to the material, e.g.,steel, as well as dimensional tolerances being held before heat treat.Conventionally used carburized steel grades for gears include SAE 8620,4320, and 9310. The grade refers to the chemical composition of thematerial including carbon, manganese, nickel, chromium, molybdenum, etc.

In addition, the process for making the steel can have significanteffects on the service life of the gear and heat treat response of thematerial. For example, a regular grade of steel (e.g., SAE 8620) with nospecial processing and a bearing quality/aircraft quality grade which isvacuum degassed while the metal is still molten to remove nonmetallicinclusions from the metal. Additionally, there are strand cast and ingotcast processes which can yield different properties. After the metal iscast and rolled into usable product shapes, the final thermal processmakes a significant difference in the dimensional response when it iscarburized as a gear.

Gears can be “core treated” before carburizing in an attempt to achievea stress-free state and minimize any dimensional change during thecarburizing process. This involves making a gear blank that has someextra stock on it, but without any gear teeth formed therefrom. The gearblank can be heated, quenched in oil, and reheated to “temper” or softenthe blank so that the final shape can be cut with gear teeth.

The carburizing process also relieves any residual stress in theworkpiece material because carburizing involves heating the material toabout 1700° F. As the workpiece is heated, any stresses in the workpiececause localized portions to change shape as the stress is relieved. Thisis a major source of dimensional change. Therefore, it is most desirableto have a stress-free workpiece before carburizing.

There are also quality concerns related to the machining quality priorto heat treat. Some gears, especially large gears that have at least an8 inch outer diameter, are restored to a usable dimensional state by aprocess called “press quenching.” The other and more common process iscalled “free quenching” or simply quenching in oil. Press quenchinginvolves heating the part to about 1550° F. and then moving it to apress with special fixtures that clamp the part flat while it is stillred hot and then dispenses oil over the part until it is immersed inoil. This process can only restore the part to a flat condition on thegear face and reset the axial dimension to a pre-heat treat state.

In the carburizing process, parts can droop, distort, bend, “potatochip”, etc., because the steel is soft at higher temperatures and sags.This becomes a real problem especially on larger gears (e.g., gearshaving large outer diameters). In general, the critical dimensions forthe part are those that contact the press and they must be held at±0.001″ prior to heat treat in order to obtain acceptable results afterheat treat.

In block 106, another process control in method 100 is the optimizationof stock removal. There are at least four ways this is achievedincluding 1) targeted size (hobbing), 2) range of growth (heat treat),3) minimized distortion (heat treat), and 4) amount and number of passeson a grinder. As for targeted size, the targeted pitch diameter of thegear or workpiece establishes the amount of stock to be removed by thegear grinder. Not only should the target avoid removing more stock thannecessary, but also the applied tolerances should support reducingvariation in stock removal from tooth to tooth, piece to piece, andoperation to operation.

The growth, or range of growth, of the workpiece through carburizing andhardening can result in excess stock being removed per flank. In somecases, however, there may not be enough stock removed to form a “good”quality gear if the workpiece does not grow as expected. In general, agear that has a diameter of 4 inches may not change. A gear having adiameter less than 4 inches will likely shrink, whereas a gear having adiameter greater than 4 inches will likely grow. For example, a gear maygrow from about 0.001-0.0015 inches per inch of diameter depending onthe SAE grade for larger gears through the carburizing process. Toensure proper growth, workpiece material is specified at the time ofpurchase and the machining process is usually CNC controlled. The growthor shrinkage resulting from the heat treat process is identified andcompensated for during the grinding process. As an example, if a gearhas a finished diameter of 12.0000 inches, the expectation would be forthe gear to grow about 0.012 inches as a result of the heat treatcarburize process. When the gear teeth are hobbed, the growth of thegear is compensated for by cutting the pitch diameter at about 11.9940inches. After heat treat, the gear would be 12.0060 inches after growingby 0.012 inches. The gear is then ground to 12.0000 inches on thegrinder.

The workpiece or gear can be subject to distortion through carburizingand hardening. As such, the process control method 100 takes intoaccount distortion, which can be referred to as taper, crown, hollow, orprofile variation of the workpiece. Failure to take distortion intoaccount can cause excess stock removal per flank when the workpiece isground. As an example, assume a gear having a final diameter of 12inches is desired. When the gear is first machined, the pitch diametermay be 11.994 inches as cut and the outside diameter of the gear isconcentric and cylindrical. Taper occurs when the pitch diameter atopposite ends of the gear tooth differ. In this example, following heattreat, one end of the gear may have a diameter of 12.006 inches and theother end is 12.001 inches. When grinding begins to cut the pitchdiameter to 12.000 inches, the grinder contacts the end having thediameter of 12.006 inches first. Depending on the setup of the grinder,excess stock may be removed during the first pass which can cause thegear to suffer burns. This can be taken into account by measuring theworkpiece or gear before the first grind pass. New equipment can accessthe stock condition and choose an appropriate course of action forgrinding the gear to specification. In addition, distortion can beminimized by selecting heat treat variables that make the process morerobust to variation.

Lastly, optimizing stock removal includes determining the number ofpasses a grinder will make on a workpiece and how much stock is removedduring each pass. The purpose of this step is to determine the amount ofstock to be removed during a pass and the number of passes whicheconomically grinds the workpiece without “loading” the grinding wheelor having the grinding wheel “rub” the workpiece excessively therebycreating undue friction and/or heat in the work zone. Once the amount ofstock removal and number of passes is determined, these quantities areprovided to the CNC gear grinder.

The process control method 100 of FIG. 1 includes block 108 whichapplies stock divide technology on each workpiece prior to grinding.This technology minimizes the risk of unequal stock removal during the360° rotation of the workpiece. On a CNC grinder, for example, thegrinding wheel makes contact with a predefined number of spaces betweengear teeth. Calculations are made by the grinder to determine thecenterline of a tooth space, thereby balancing stock removal fromside-to-side.

In block 110, the amount of coolant flow directed into the “work zone”during the grinding operation is determined. The “work zone” is definedas the location or area in which the grinding wheel contacts and removesmaterial from a workpiece. Coolant is important to removing heat betweenthe grinding wheel and the workpiece during the grinding operation. Whenthe maximum amount of coolant flow is directed into the “work zone,” therisk of surface tempering is reduced. Different types of coolant can beused such as, for example, oil, synthetic, or water based. During aconventional grinding cycle, between approximately 90-120 psi of coolantis dispensed into the “work zone.” The pressure can be different forother grinding cycles. To support this amount of coolant pressure, thecoolant system can have a 500 gallon capacity with temperature controlsand a filtering system.

The selection of the grinding wheel in block 112 of the process controlmethod 100 is another important consideration when manufacturing aquality part. Hardness and grain size are two criteria used forspecifying grinding wheels. Other specifications for selecting grindingwheels include abrasive type, abrasive size, grade or hardness,structure, and bond. The purpose or goal is to correlate the resultantwheel breakdown to the wheel dressing parameters and required cycletime. In other words, the purpose of the grinding operation is to removematerial from the workpiece with abrasive action for achieving tighttolerances and fine surface finishes. The grain structure of thegrinding wheel acts as a cutting tool for forming very small chips.Grinding wheels are self-sharpening tools due to the friability of thebond between grains. Friability is the ability to fracture underpressure so that as the cutting edges become dull, the grain breaks offand exposes new, sharp cutting edges. However, the self-sharpeningphenomenon is supplemented with regular wheel dressing for qualityassurance. As one skilled in the art understands, grinding wheels mustbe dressed periodically after grinding a workpiece so that material fromthe workpiece can be removed from the grinding wheel and the uniformityacross the surface of the grinding wheel can be maintained.

Another consideration in the process control method 100 is theoptimization of wheel dressing parameters. In block 114, dressingparameters include the frequency of dressing, the amount, and the rateat which the wheel is cleaned, sharpened, and “made like new” for thenext workpiece. Wheel dressings can occur at different frequencies. Forexample, as a larger workpiece is being ground, the program can pause sothat the grinding wheel can be dressed. In other instances, severalparts can be ground before the wheel is dressed. During a dressing, anyamount of material can be removed from the grinding wheel. As anon-limiting example only, about 0.002-0.005 inches of material can beremoved from the grinding wheel. The rate at which the grinding wheel isdressed refers to the speed and feed used when dressing the wheel. A newgrinding wheel can be any size depending on the type of application itis being used for. As another non-limiting example, a new grinding wheelmay have a diameter of 14 inches. The same grinding wheel may still befully functional in the range of 10-11 inches in diameter. After thegrinding wheel is dressed below a certain diameter, however, thegrinding wheel must be replaced with a new one.

In block 116, the process control method 100 also includes the detectionand/or prevention of grind burn on a workpiece. The effects of grindburn have been described above. In conventional process control methods,nital etching has been the industry standard for detecting grind burns.However, as described above, there are limitations and disadvantagesassociated with nital etching. As such, the present invention providesan alternative to nital etching that overcomes the limitations anddisadvantages thereof.

With reference to FIG. 2, a method of detecting and/or preventing grindburn is provided. The method 200 includes acoustic emission detection202, eddy current testing of the grinding wheel 204, and swarf analysis206. Each of these methods can be performed individually or incombination with one another. In one embodiment, for example, method 200may be performed by acoustic emission detection only. In a differentembodiment, however, both acoustic emission detection 202 and swarfanalysis 206 are performed. In another embodiment, all threedetection/prevention methods can be carried out. As will be described infurther detail, each method can detect and/or prevent grind burns on aworkpiece by analyzing or testing a different component of the grindingcycle. As such, each method is independent of the others, but a completeanalysis considers all three methods.

With reference to FIGS. 2A, 3, and 4, an exemplary embodiment isprovided for detecting and/or preventing grind burns on a groundworkpiece via an acoustic emission process 202. For illustrativepurposes only, an exemplary setup 300 (FIG. 3) is shown of a gear 308during a grinding cycle in which a grinding wheel 304 cuts material fromthe gear 308 to form the profile of a gear tooth 312. The acousticemission process 202, however, is not limited to grinding gears. Theprocess can also be used on other workpieces that are ground and subjectto grind burns.

The grinding cycle can occur in a test facility 300 in which a grinderor grinding machine 302 includes a motor or the like for driving thegrinding wheel 304. Depending on the size of the workpiece, the grindingwheel can be any size. For example, gear teeth for any type of gearincluding steering gears, range gears, power take-off (PTO) gears, etc.for assembling in a transmission can be formed using the grinding wheel.In FIGS. 3 and 4, the grinding wheel is about 14″ in diameter. Thegrinding wheel can also be made of any material suitable for grindinggears such as aluminum oxide, Cubic Boron Nitride, Silicon Carbide, andSynthetic Diamond. One skilled in the art will appreciate that grindingwheels can be made of other materials as well.

During a grinding cycle, the gear 308 is fixed or secured in place by aworkpiece holder 310. The workpiece holder 310 is able to rotate 360° sothat each tooth 312 can be ground by the grinding wheel 304. Moreover,the grinding wheel 304 rotates in a direction indicated by arrow 306 andremoves material from the flank of the tooth 312. Due to the frictioncaused between the gear tooth 312 and grinding wheel 304, a significantamount of heat is generated therebetween. To cool the gear tooth 312being ground, a coolant dispenser 314 directs coolant 406 (FIG. 4) ontothe gear tooth 312 during the grinding cycle. As noted above, differenttypes of coolant can be used including oil, synthetic, or water based.

The acoustic emission method 202 of detecting and preventing grind burnsfrom forming on the ground gear is performed during the grinding cycle.Unlike conventional methods for detecting grind burns such as nitaletching which can only detect grind burns after grinding, the acousticemission method 202 allows for immediate detection of grind burns duringthe grinding cycle. As a result, the acoustic emission method 202provides the opportunity to detect grind burns before they form on thegear tooth 312. To do so, an acoustic emission sensor 316 is positionednear the grinding wheel 304 and gear 308. In the illustrated setup 300of FIGS. 3 and 4, the sensor 316 is spaced about 12 inches from thegrinding wheel 304 and gear 308. The distance between the sensor 316 and“work zone,” i.e., the area immediately surrounding the grinding wheel304 and gear 308, can depend on the type of sensor being used. Onenon-limiting example of an acoustic emission sensor that can be used ismade by Dittel (http://www.dittel.com). The acoustic emission sensor 316is coupled to a processor 400 via a cable 318. The processor can includeseveral connectors 404 that are adapted to receive cables, wires, andthe like. Although only one acoustic emission sensor 316 is depicted inFIGS. 3 and 4, it is possible for a plurality of sensors to beimplemented in the test setup 300. The processor 400 can receive datafrom the sensor 318 and results or computations can be displayed on adisplay monitor 402.

Before or during a grinding operation, the acoustic emission sensor 316is positioned adjacent to the grinding wheel 304 and gear 308. Theacoustic emission sensor 316 can be positioned automatically (e.g., arobot or automation system moves the sensor to different positions andorientations) or manually. For example, before a grinding operation, atest operator can position the sensor 316 in a desired location. Oncethe sensor 316 is in the desired position, the grinding wheel 304 beginsrotating in the direction indicated by arrow 306. As this is done, agear tooth 312 is brought into contact with the wheel 304 and materialis removed from the tooth 312.

As material is removed from the gear during the grinding operation, anacoustic emission is produced between the interaction of the grindingwheel 304 and gear tooth 312. The emission consists of measurablefrequencies primarily in the ultrasound range. As known by the skilledartisan, emissions that fall in the range of human hearing producefrequencies between about 16 Hz to 20 kHz. In the ultrasound range,frequencies are produced between 20 kHz and 1 GHz. Acoustic emissionscan produce measurable frequencies between about 20 kHz and 2 MHz. Theacoustic emission sensor 316 is able to detect the emissions during thegrinding operation and communicate the measured emissions to theprocessor 400. The processor 400 can analyze the measured emissions anddisplay the results on the display monitor 402.

An advantage of measuring the acoustic emission during the grindingcycle is that grind burns can be detected as the gear is being ground.It has been found that a certain level or threshold of acoustic emissionis directly correlated with grind burns forming on a gear tooth.Moreover, when the acoustic emission remains below or does not exceedthe threshold, grind burns do not normally form on the gear tooth beingground. As a result, the acoustic emission process 202 of FIG. 2includes measuring the acoustic emission with the sensor 316 (block 210)and comparing the measured acoustic emission to a threshold (block 212).The measured acoustic emission can be displayed on the display monitor402 along with the threshold acoustic emission.

In block 214, based on the results of block 212, a determination is madewhether grind burns may be forming on the gear or workpiece. Asdescribed above, one way in which grind burns form is when too muchmaterial is removed from the gear or workpiece. When excessive materialis removed from the gear or workpiece, the acoustic emission produced isgreater than when a normal or desired amount of material is removed. Anancillary component of the acoustic emission process 202 is determiningwhat level or value of acoustic emission equates to when a normal ordesired amount of material is removed during the grinding operation. Asa non-limiting example, when the desired amount of material is removedfrom the flank of the gear tooth 312, the sensor 316 may measure 10 dBof acoustic emission. Based on this “normal” or “targeted” acousticemission, a threshold acoustic emission is determined such that once themeasured acoustic emission is equal and/or greater than the thresholdacoustic emission, a determination is made that conditions are favorablefor grind burns to form. This real-time detection method is advantageousover nital etching and other conventional grind burn detection methodsbecause each gear is being tested and results are immediately known. Themachine operator can observe the display monitor 402 during the grindingoperation and manually shut down the grinding machine 302.Alternatively, the system can be automated such that once the measuredacoustic emission exceeds the threshold acoustic emission for a periodof time the grinding operation is suspended. A warning system can alsobe provided that produces an audible and/or visual alert when themeasured acoustic emission exceeds the threshold. The alert can bedisplayed on the display monitor 402, for example.

In this example, 10 dB refers only to the amplitude of the noise signal.Besides the amplitude, there may be a frequency shift or other soundmeasurement that can be made for identifying potential grind burns. Inaddition, one skilled in the art can appreciate other ways in whichautomated alarms and warning systems can be implemented for detectingand/or preventing grind burns forming on a workpiece.

As described above, the acoustic emission process 202 is a real-timetest procedure for detecting and/or preventing grind burns from formingon a ground workpiece. A different embodiment for detecting and/orpreventing grind burns from forming on a workpiece during a grindingoperation is the eddy current testing process 204. Eddy current testinganalyzes the grinding wheel subsequent to a grinding operation anddetermines whether the grinding wheel is properly dressed. Throughvarious testing and studies, it has been determined that almost 15% ofdamage associated with the grinding of a workpiece is attributable tothe dressing condition of the grinding wheel. As such, the eddy currenttesting process 204 provides another method for detecting and/orpreventing grind burns from forming on a ground workpiece.

With respect to the illustrative embodiment of FIGS. 2B, 5, and 6, theeddy current testing process 204 is performed after a gear tooth 510 ofa gear 508 is ground. In the setup 500 of FIGS. 5-6, the gear 508 iscoupled to a workpiece holder 600. The workpiece holder 600 ispositioned near a grinding machine 602, which includes a grinding wheel502 powered by a motor or the like (not shown). The grinding wheel 502can have any diameter and made of any material. As described above, onenon-limiting example of a grinding wheel 502 has a diameter of about 14inches and is made from aluminum oxide. The contour of the grindingwheel can also be either concave or convex depending on the shape of thegear tooth being formed.

After the grinding operation, the grinding wheel 502 is moved away fromthe gear 508 and, in block 216 of FIG. 2, an eddy current probe 504 isplaced in contact with the grinding wheel 502. An example of an eddycurrent probe that can be used in this setup is a Statograph® 6.421Probe available at www.foerstergroup.com. The probe 504 can be connectedto a meter or scope 604 by a cable 506. The meter or scope 604 can havea graphical display 606 with user control buttons 608 for operating themeter or scope 604. An example of a meter or scope that can be used foreddy current testing can also be found at www.foerstergroup.com.

During the grinding operation, material that is removed from theworkpiece can become embedded in the grains of the grinding wheel 502.In the case of a gear, which can comprise up to 98% iron, gear filingsthat are removed from the gear become embedded in the grinding wheel. Iftoo much iron is embedded in the grinding wheel and the wheel is notdressed, further grinding with the same grinding wheel can generategrind burns on the gear tooth. In the case of an aluminum oxide grindingwheel, for example, the eddy current testing process 204 can detect apresence of iron embedded in the grinding wheel by creating an electricfield in the grinding wheel (see block 218). In the eddy current sensor504, there can be a primary and secondary coil (not shown). The primarycoil can pass an electric signal such as current to the grinding wheel.As one skilled in the art understands, when an alternating current flowsin a coil, the magnetic field of the coil can produce circulating eddycurrents in close proximity to the conducting surface. In the presentembodiment, if iron is embedded in the wheel, the electric field createsa reverse or induced signal in the grinding wheel. Iron is magnetic, andthe induced eddy current has magnetic and resistive properties. As such,the presence of iron is detectable via the magnetic component. In block220, the secondary coil receives the induced signal and the probe 504measures the strength of the signal. The measured signal can bedisplayed on the graphical display 606 in any form known to the skilledartisan (e.g., in graphical format, digital format, etc.).

In block 222, the measured signal is compared to a threshold eddycurrent value. The threshold can be established in a plurality of ways.One such way for determining the threshold is to perform the eddycurrent testing process 204 on a grinding wheel which has never beenused. In such a case, there should be no induced eddy current signal andtherefore the measured induced signal would measure 0 volts, forexample. While it is possible for the measured signal to have differentunits such as dB, voltage is easy to measure and/or convert to fromother units. In one aspect, if the threshold is 0 volts, any presence ofiron embedded in the grinding wheel will create a measurement greaterthan the threshold. However, a small presence of iron embedded in thegrains of the grinding wheel can be unavoidable and does not alwaysresult in grind burns. As such, in a different aspect, a thresholdgreater than 0 volts is established with the understanding that agreater presence of iron is allowed to be embedded in the grindingwheel. In one non-limiting example, any measured signal less than 0.25mV is considered to be satisfactory for purposes of reusing the grindingwheel without dressing the wheel. This “determination” step comprisespart of block 224 of FIG. 2.

In block 222, if the measured signal is greater than the threshold, thismay not suggest that the gear has suffered surface tempering. Instead,it may indicate that continued use of the grinding wheel withoutdressing the wheel will provide favorable conditions for damaging thegear. Thus, if the grinding wheel needs to be dressed, about 0.002″ ofmaterial is removed from the diameter of the grinding wheel and thewheel is ready for further grinding operations. The eddy current testingprocess 204 therefore provides an additional method of detecting and/orpreventing grind burns from forming on a ground workpiece and ensuringthat the grinding wheel is adequately dressed.

In FIG. 2C, another embodiment of a method is provided for detectingand/or preventing grind burns from forming on a workpiece during agrinding operation. A swarf analysis process 206 can detect and/orprevent grind burns by analyzing the swarf produced during the grindingoperation. Swarf is referred to as the material removed by a cutting orgrinding operation. Before the swarf can be analyzed, it must first becollected during the grinding operation. One skilled in the art canappreciate the many ways swarf can be collected. In one aspect, theswarf can be collected by a magnet. In a different aspect, filter paper700 can be used as illustrated in FIGS. 7 and 8. Filter paper is asemi-permeable paper barrier that is typically used for separating finesolids from liquids or air.

In the setup illustrated in FIG. 8, the filter paper 700 is positionedat the base of a workpiece holder 800. The filter paper 700 is spreadaround the circumference of the base and is disposed beneath a gear 802which is coupled to the holder 800. The workpiece holder 800 is able torotate 360° so that the entire gear can be ground. In block 226 of FIG.2C, a gear tooth 804 of the gear 802 is ground by a grinding machine(not shown). During the grinding operation, swarf 806 is produced. Asthe swarf 806 is produced, it falls onto and is collected by the filterpaper 700 (block 228 in FIG. 2C). With reference to block 230, after thegrinding operation, the collected swarf 806 is inspected for indicia ofgrind burn.

Since the workpiece holder 800 is able to rotate 360°, both the gear 802and filter paper 700 also rotate the same distance. Therefore, when agear tooth 804 at the location 814 of the gear 802 is ground, swarf 806is collected at or near location 810 of the filter paper. In thisembodiment, location 814 is disposed almost directly above location 810of the filter paper 700. Similarly, when a gear tooth 804 at location812 of the gear 802 is ground, swarf 806 is collected at location 808 ofthe filter paper 700. Again, location 812 of the gear is disposed almostdirectly above location 808 of the filter paper 700. Thus, when a pieceof swarf 806 is inspected from location 808 of the filter paper 700, theinspector is able to correlate the location of the filter paper 700 withthe location along the gear 802 from which the swarf came. While this isa general and non-limiting example of how collected swarf is linked tothe gear tooth from which it is removed, other processes known to theskilled artisan can be used.

In FIG. 9, one example of how the swarf 806 is inspected in block 230 isshown. A camera or microscope 900 having a base 904, a lens 902, andseveral user controls 906 can be used for the inspection. An example ofsuch a camera 900 is a Keyence Digital Microscope VHX 100 (seewww.keyence.com/products). The camera should include a highmagnification feature so that swarf 806 can be analyzed up to 100×, ormore advantageously at about 173×. Indicia of grind burns can includechanges in color and/or thickness in the collected swarf 806. Forexample, a gear tooth that has a grind burn can have a dark gray, blue,or black appearance, whereas a gear tooth free of grind burns will havea light gray or light brown appearance. The thickness of collected swarfdoes relate to the grinding force, and therefore a chip thickness thatexceeds a predetermined maximum thickness threshold would indicate achange or problem in the grinding process. The threshold can bedetermined, for example, by mathematical modeling as described in anarticle entitled “Grinding Force and Power Modeling based on ChipThickness Analysis” by Rogelio Hecker et al. in The InternationalJournal of Advanced Manufacturing Technology, Vol. 33, Nos. 5-6, June2007. If an inspection identifies that there is a grind burn on a geartooth, the inspector is able to correlate the location of the collectedswarf with the location on the gear from where the swarf was removed.Further inspection and analysis of the gear can then determine whetherthere is any grind burn and, if so, the severity of the grind burn. Theswarf analysis process 206 therefore provides another means fordetecting and/or preventing grind burns.

In an advantageous process control method 100, grind burns are detectedand/or prevented by performing the acoustic emission process 202, theeddy current testing process 204, and the swarf analysis process 206.Rather than performing only one of the detection and/or preventionmethods, each gear or workpiece that is ground on a grinding machine isanalyzed and monitored for possible grind burn. In this embodiment, theacoustic emission process 202 monitors and detects grind burns during agrinding operation. After the grinding operation, swarf that iscollected during the grinding operation is then analyzed under a cameraor microscope as described above.

The swarf analysis process 206 can be performed regardless of whetherthe acoustic emission process 202 detected potential grind burns. Assuch, the swarf analysis process 206 provides an additional safety netto enhance the quality of parts being manufactured. Moreover, each partbeing manufactured is tested and analyzed, which provides advantagesover conventional detection methods. In particular, if grind burns aredetected using either the acoustic emission process 202 or swarfanalysis process 206, the entire process control method 100 can befurther analyzed to determine whether there are problems in the materialof the workpiece, grinding wheel, etc. Since conventional detectionmethods are unable to provide real-time feedback, many additionaldefective parts are made before a problem is identified and changes canbe implemented in the process control method.

In addition, the eddy current testing process 204 further identifiesopportunities or conditions favorable for producing grind burns byanalyzing the grinding wheel after the grinding operation. Therefore,this embodiment which includes the acoustic emission process 202, theeddy current testing process 204, and the swarf analysis process 206provides the most complete and effective means for detecting and/orpreventing grind burns from forming on a ground gear or workpiece.

While exemplary embodiments incorporating the principles of the presentinvention have been disclosed hereinabove, the present invention is notlimited to the disclosed embodiments. Instead, this application isintended to cover any variations, uses, or adaptations of the inventionusing its general principles. Further, this application is intended tocover such departures from the present disclosure as come within knownor customary practice in the art to which this invention pertains andwhich fall within the limits of the appended claims.

1. A large-scale production process for making gears, comprising: (a)establishing tolerances for the amount of material removed from stock toform the gears; (b) selecting a grinding wheel that maximizes cuttingefficiency and requires infrequent dressing; (c) determining the numberof passes the grinding wheel will make to remove material from the stockduring a grinding operation; (d) establishing the amount of coolant flowto be dispensed during the grinding operation; (e) grinding the stockwith the grinding wheel during the grinding operation; (f) making gearsout of the stock; (g) determining a condition favorable for generatinggrind burn on the gears is present before, during, and after thegrinding operation; and wherein, when the condition favorable forgenerating grind burn is determined, adjusting one or more parameters ofthe grinding operation to eliminate the condition and then repeatingsteps (e)-(g).
 2. The method of claim 1, wherein the determining stepcomprises: positioning a sensor near the stock and grinding wheel beforethe grinding operation; measuring an acoustic emission signal with thesensor; and comparing the measured acoustic emission signal to athreshold; wherein, if the measured acoustic emission signal exceeds thethreshold, a condition favorable for generating grind burn is inferred.3. The method of claim 2, wherein if the measured acoustic emissionsignal exceeds the threshold, inferring excessive material is removedfrom the stock during the grinding operation.
 4. The method of claim 1,wherein the determining step comprises: placing a probe in contact withthe grinding wheel before the grinding operation; measuring an inducedsignal in the grinding wheel with the probe; and comparing the measuredinduced signal to a threshold; wherein, if the measured induced signalexceeds the threshold, a condition favorable for generating grind burnis inferred.
 5. The method of claim 4, wherein if the measured inducedsignal exceeds the threshold, inferring the grinding wheel needs to bedressed or replaced.
 6. The method of claim 4, wherein if the measuredinduced signal is less than the threshold, inferring the grinding wheelis properly dressed.
 7. The method of claim 4, wherein after the placingstep, the probe induces an electric field in the grinding wheel.
 8. Themethod of claim 7, further comprising detecting the induced signal withthe probe.
 9. The method of claim 1, wherein the determining stepcomprises: collecting swarf removed from the stock during the grindingoperation; and inspecting the collected swarf for an indication of grindburn; wherein, when the indication is a discoloration or change inthickness of the collected swarf, a condition favorable for generatinggrind burn is inferred.
 10. The method of claim 9, further comprisinginferring grind burn on the gear when the collected swarf appearssubstantially dark gray, blue, or black.
 11. The method of claim 9,further comprising using filter paper or a magnet to collect the swarf.12. The method of claim 9, further comprising placing filter papersubstantially beneath the stock.
 13. The method of claim 12, furthercomprising correlating the location of the collected swarf on the filterpaper with the location on the stock from which the swarf is removed.14. The method of claim 1, wherein, when the condition favorable forgenerating grind burn is determined, the grinding operation issuspended.
 15. The method of claim 1, wherein, when the conditionfavorable for generating grind burn is determined, a warning signal isproduced.
 16. The method of claim 1, wherein the parameters comprises atleast one of steps (a)-(d).